This invention relates to a stent for a biliary, urinary or vascular system, in particular a stent of shape memory alloy (SMA) to be placed in a pathologic stricture portion of the biliary, urinary or vascular system in the human body.
Urethral stents made of shape memory alloy have been developed and tested in small clinical trials. However such prior art stents have failed to provide a useful, practical treatment of prostatic obstruction.
An example of prior art stents is shown in FIG. 9, which comprises a spiral coil 2 with a proximal tapering end 1 and a distal hinge 7 for removal. In this context, an end of a stent which is to extend furthest into a patient's body is referred to as a "proximal" end, while the other end as a "distal" end. Another type of stent, which is not illustrated, is a cross woven cylindrical stent of, for example, 12 mm in diameter.
Such prior art stents have Austenite finished temperatures (Af temperatures) of about 48.degree. C. or 36.degree. C., respectively. Thus, the stents transform in shape at their respective Af temperatures.
An SMA stent having an Af temperature of 48.degree. C. must be specially designed so that it does not burn the mucosa of the human body. In fact, the mucosa of the human body will be burned at temperatures of about 43.degree. C. or above.
Another SMA stent having an Af temperature of 36.degree. C. is more sensitive to temperature fluctuations and liable to change its shape inadvertently. For example, it could change its shape during transportation or storage because an ambient temperature in package or box is easy to be heated up to about 36.degree. C., or perhaps in a doctor's or nurse's hand due to the human body temperature.
Furthermore, mesh stents require the assistance of an endoscope or other special accessory tool under ultrasonic or fluoroscopic control for accurate positioning and removal.
A stent may have to be removed for a variety of reasons, such as for example, once it has performed its function and is no longer necessary, or perhaps if it becomes encrusted, or if it migrates from its desired location such as into a bladder cavity. Thus, it can be very time consuming to use such stents. Also, they may occasionally traumatizes a uretra, a blood vessel or a bile duct, which may lead to bleeding or intolerable pain for patients.